Flush stdin from within chroot archive

This makes sure that we don't buffer in memory and that we also flush
stdin from diff as well as untar.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

Conflicts:
	pkg/chrootarchive/diff.go

chrootarchive/archive.go

@@ -6,7 +6,6 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -40,9 +39,7 @@ func untar() {
 		fatal(err)
 	}
 	// fully consume stdin in case it is zero padded
-	if _, err := ioutil.ReadAll(os.Stdin); err != nil {
-		fatal(err)
-	}
+	flush(os.Stdin)
 	os.Exit(0)
 }